Heat treatment

• Phase diagram – Iron-carbon diagram• TTT diagram for carbon steel• Heat Treatment processes

•Annealing•Normalising•Tempering •Surface hardening

Heat treatment (HT) basicsHT is defined as a process of controlled heating and cooling procedures performed to effects the microstructural changes in a material, which in turn affect its mechanical properties without change in dimensions and geometry of parts.

The basis of changing properties thro HT is essentially through modification of internal structure of the material which characterized in term of phase present, their size, shape and distribution in the material.

HT is used to enhance service life, easy for fabrication – machining and forming.

It can be done during the process – process HT (annealing/hotworking) or after the process – HT (annealing)

Iron-carbide phase diagram

Iron-carbide phase diagram

Part of the equilibrium phase diagram for the Fe-C system

6.67

25

L

γ + L

γ + Fe3C

α + Fe3C (Cementite) Fe3C

γ, Austenite

γ + L

α, Ferrite

6.67

25

L

γ + L

γ + Fe3C

α + Fe3C (Cementite) Fe3C

γ, Austenite

γ + L

α, Ferrite

6.67

25

L

γ + L

γ + Fe3C

α + Fe3C (Cementite) Fe3C

γ, Austenite

γ + L

α, Ferrite

6.67

25

L

γ + L

γ + Fe3C

α + Fe3C (Cementite) Fe3C

γ, Austenite

γ + L

α, Ferrite

TTT – diagram – quenching cycle

Time-Temperature path on isothermal Transformation diagram

TTT – diagram – 0.8% C steel

Heat treatment ProcessesPrecipitation hardeningAnnealingNormalizingTemperingSurface hardeningSelective surface hardening

Precipitation hardeningFine particles that block movement of dislocation and thus strengthen and harden the metalCommon in non ferrous alloysThe alloy should have two phases at RT, when heated to a temperature it should dissolve the otherSteps involved

Solution treatmentQuenchingPrecipitation treatment

Annealing, Normalising & Tempering

To reduce hardness, remove residual stress, improve toughness, restore ductility Annealing – heating to a predefined temperature (above upper critical temperature), holding it for some time and then furnacecooling/inert atmosphere which forms coarse pearliteNormalising - heating to a predefined temperature (above upper critical temperature), holding it for some time and then air cool. Normalising materials are hard than annealingSpheroidization anneal – for more than 6% CTempering – To decrease the internal stress a secondary heating below the lower critical temperature of 723 C and then cooling in a prescribed rate

Surface Hardening Thermochemical treatment

Carburizing – heating in a carbon rich environment so that C is diffuse into the surface.Nitriding – Nitrogen diffuse into surfaceCarbonitriding – Carbon and nitrogen richChromizing – high temperature , longer timeBath

Gas carburizing Pack carburizingLiquid carburizing

Flame hardening –oxy acetylene flameInduction hardening - ( permeability of material, resistivity)Laser beaming hardeningElectron beam hardening

Selective Surface hardening method

At a glance

HTPhase diagram for Fe-CTTTHeat treatmentSurface treatment